1. Field of the Invention
The present invention relates to a character display apparatus which displays a character image.
2. Background of the Invention
A character display apparatus generates a character image from font data according to character information including a font type, a character code, and a character size. A character image is generated from outline font data which are vector data showing the appearance of a character, or from stroke font data which are vector data describing the frame of a character. More specifically, when displaying a character image on the screen of a display device, a character display apparatus carries out scaling of vector data as mentioned above according to the size of the character image to be outputted on the above-mentioned screen and fills the appearance or frame defined by the above-mentioned vector data with a color so as to generate the character image.
Generation of a character image is a process of converting predetermined vector data into the character image having one of a wide variety of sizes, as mentioned above, and has a high processing load and consumes many CPU resources. Therefore, in order to achieve an improvement in the speed of generation of a character image, there has been proposed a technology for storing vector data used for generation of a character image in a cache memory and reusing the vector data stored in the cache memory when generating a new character image.
For example, an apparatus disclosed by patent reference 1 stores vector font data for each and every font type in a storage unit, such as a large-volume hard disk drive, and is also provided with a first cache memory for storing generated character images, and a second cache memory for storing vector font data used for generation of character images.
When generating a character image, the apparatus searches through the first cache memory after receiving a font type and a character size as inputs thereof, and, if the apparatus cannot search for a character image corresponding to the input data, further searches through the second cache memory. If vector font data corresponding to the input data are stored in the second cache memory, the apparatus generates a character image by using the vector font data. In contrast, unless vector font data corresponding to the input data are stored in the second cache memory, the apparatus extracts the above-mentioned vector font data from the storage unit and then expands the vector font data, and then generates a character image.
Thus, because the apparatus according to the invention disclosed by the patent reference 1 is provided with the first cache memory for storing character images, the apparatus can output a character image whose font type and size match the input data at a high speed. Furthermore, because the apparatus has the second cache memory for storing vector font data, when generating a character image whose font type matches the input data, but whose size does match the input data, the apparatus can acquire corresponding vector font data without accessing the storage unit.
By doing in this way, even when the character size of an on-screen character is changed, if corresponding vector font data whose font type matches that of the character exist in the second cache memory, the conventional apparatus can generate a character image at a high speed without accessing the storage unit.    [Patent reference 1] JP,6-118936,A
In embedded systems in which, in recent years, an improvement has been achieved in the resolution of a display device, there are frequent cases in which on-screen characters are color-coded in order to improve the visibility of display information. That is, some character images which have the same font type and the same character size, but have different foreground colors and different background colors are generated.
In this case, in accordance with a structure, as shown in the patent reference 1, of storing a final character image in a cache memory, when a character image to be newly generated has the same font type and the same character size as the previous character image, but has a different foreground color and a different background color, the character image stored in the cache memory is not reused. Therefore, in this case, the storage unit has to be accessed and the character image has to be generated, and no improvement can be achieved in the speed of the character image generation processing.
Furthermore, conventionally, an improvement is achieved in the speed of data input by acquiring vector font data from a cache memory, though it is necessary to, after acquiring the vector font data, carry out a high-processing-load process of converting the vector font data into a character image. A problem is therefore that even if data acquisition is carried out at a high speed, an improvement cannot be achieved in the speed of generation of a final character image, depending upon the processing performance of the conventional apparatus.